I came across some diesel reservoirs in Brussels, Belgium, that appear to have overflow lines with a vented siphon design, see photo below.
What's the purpose of this design? I assume the vent is to prevent the overflow line from siphoning the fuel down to the level of the inlet of the overflow line, but why use a siphon in the first place? Why would the overflow line not start at the very top of the tank and use a 90° elbow to go down?
It put the max liquid line higher. With the elbow the liquid has to reach to the elbow instead of just the hole in the wall.
The hole itself cannot be put at that height because that's where the roof starts, much easier to put the hole lower down where sealing it is much easier.
In this design it look like that gives them another 50 cm of liquid level.
If the intake of the overflow was at the bottom of the tank then it will also isolate the gas portion of the tank from the outside air. Which can be important with liquids that off-gas flammable vapours.
